It is probable, though the above appear to exhibit the primary types of nervous systems, that others exist of an intermediate nature, with which future investigators may render us better acquainted[12]: but as our business is solely with that upon which insects in this respect have been modelled, without expatiating further in this interesting field, I shall therefore now confine myself to them.
We have before seen[13] that the nervous system of insects belongs to the ganglionic type: but it requires a more full description, and this is the place for it. It originates in a small brain placed in the head, and consisting almost universally of two lobes, sometimes extremely distinct. It is placed over or upon the œsophagus or gullet, and from its posterior part proceeds a double nervous chord, which embracing that organ as a collar dips below the intestines, and proceeds towards the anus, forming knots or ganglions at intervals, in many cases corresponding in number with the segments of the body, and sending forth nerves in pairs, the ramifications of which are distributed to every part of the frame. In the perfect insect the bilobed ganglion of the head or the brain is usually of greater volume than in the larva, and the ganglions of the spinal chord are fewer, which gives a more decided character of centricity to the whole nervous system[14]. This may be considered more particularly with respect to its substance and colour; its tunics, and parts.
I. Substance and Colour.—The nervous apparatus of insects is stated by those who have examined it most narrowly, though consisting of a cortical and medullary part, the latter more delicate and transparent than the former, to be less tender and less easy to separate than the human brain[15]. It has a degree of tenacity, and does not break without considerable tension; in general, it is clammy and flabby, and under a microscope a number of minute grains are discoverable in it, and when left to dry upon glass, it appears to contain a good deal of oil, which does not dry with the rest[16]. That of the ganglions differs from the substance of the rest of the spinal chord, in being filled with very fine aërial vessels, which are not discoverable in the latter[17]. With regard to colour, Lyonet states that the chords of the spinal marrow in the larva of the great goat-moth are of a blueish gray, and have some transparence[18]; Malpighi and Swammerdam observed that the cortical part of the ganglions of that of the silk-worm and the hive-bee had a reddish hue, while the medullary part was white[19]; Cuvier relates that the brain and the third ganglion in Hypogymna dispar, with us a scarce moth, differed in colour from all the rest, being quite white, while the others were more or less tinted, and examined under a lens appeared variegated by reddish sinuous markings, resembling blood vessels as they are seen in injected glands[20].
II. Tunics.—The coats that inclose the various branches of the nervous system in insects seem analogous to those of vertebrate animals. The first thing that strikes the eye, when these parts in a recent subject are submitted to a microscope, is a tissue of very delicate vessels, which ramify beyond the reach of the assisted sight; these are merely air-vessels or bronchiæ derived originally from the tracheæ of the animal: but besides these is an exterior and an interior tunic; the first corresponding with the dura mater of anatomists; and the other, which is the most delicate and incloses the cortical and medullary parts, with the pia mater[21].
III. Parts.—The nervous system of insects consists of the brain; the spinal marrow and its ganglions; and the nerves.
i. Brain.[22] Linné denied the existence of a brain in insects, and most modern physiologists seem to be of the same opinion. A part however, analogous to this important organ—at least in its situation, and in its emission of nerves to the principal organs of the senses, in which respect it certainly differs very materially from the upper cervical ganglion, which Dr. Virey regards as its analogue[23]—is certainly to be found in them; and as Messrs. Cuvier and Lamarck distinguish this part by the name of brain, we may continue to call it by that name without impropriety. The brain of insects, then, is distinguished from the succeeding ganglions of the spinal chord by its situation in the head, the middle of the internal cavity of which it occupies, and by being the only ganglion above the œsophagus. It is usually small, though in some cases larger than they are[24]. It consists of two lobes, more or less distinct and generally of a spherical form. In Oryctes nasicornis and Pontia Brassicæ the lobes are separated both before and behind[25]; while in the larva of Dytiscus marginalis, but not in the imago, in which there are two large hemispheres separated by a furrow, the brain is undivided[26]. Cuvier mentions the larva of a saw-fly in which this part is formed of four nearly equal spherical bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28]) the two lobes represent an equilateral triangle, the exterior angle of which terminates in several lesser spherical bulbs; in Acrida viridissima, Nepa cinerea, Clubiona atrox, and the common Louse, the lobes are pear-shaped[29].
ii. The spinal marrow and its ganglions[30]. From the posterior part of the brain of insects, but in the ground and water beetles (Eutrechina and Eunechina) from its sides below[31], issue two chords which diverging embrace the œsophagus, and dipping below it and the intestines,—a situation they maintain to the end of their course,—and in their further progress uniting at intervals and dilating into several knots or ganglions, compose their spinal marrow. This part is so named, from a supposed analogy to the spinal marrow of vertebrate animals, which however admits of some degree of doubt; yet, since it mixes the functions of that organ with those of the great sympathetic nerves, the denomination is not wholly improper, and may be retained. Though this chord is usually double when it first proceeds from the brain, and surrounds the œsophagus like a collar, yet in some insects it may be called a single chord. This is the case with that of the common louse, in which Swammerdam could perceive no opening for the transmission of the part just named[32]; if he was not mistaken in this, the brain, as well as the rest of the spinal marrow in that animal, would be below the intestines; from the figures of Treviranus it should seem that the spiders, at least Clubiona atrox, are similarly circumstanced[33]; in the cheese-maggot, which turns to a two-winged fly (Tyrophaga Casei), the chord is also single, but it has a small orifice through which the gullet passes[34]. At the union of the chords in other cases below that organ, a knot or ganglion is usually formed, and an alternate succession of internodes and ganglions commonly follows to the end. The internodes also may generally be stated to consist of a double chord, though in many cases the two chords unite and become one, or are distinguished only by a longitudinal furrow, and even where they are really distinct and separable, in the body of the insect they lie close together[35]. In the rhinoceros beetle (Oryctes nasicornis) and Acrida viridissima &c. all the internodes consist of a double chord[36]; but in many other insects numerous variations in this respect occur.—Thus in the stag-beetle the last internode is single[37]; in the caterpillar of the cabbage butterfly (Pontia Brassicæ) the five first are double, and the six last single[38]; in that of the great goat-moth (Cossus ligniperda) the three first only are double, but the others terminate in a fork[39]; in the cockroaches (Blatta) the four first, in Hydrophilus piceus the three first, and in Eristalis tenax the two first only are double, the rest being all single[40]. A singular variation takes place in Hypogymna dispar; all the internodes are single, except the second, the chords of which at first are separate, and afterwards united[41]; and, to name no more, in Clubiona atrox there is only one internode, which is single, with a longitudinal furrow[42]. In some, as in the louse, the grub of Oryctes nasicornis, and the cheese-maggot, there are no internodes, the spinal marrow being formed of knots separated only by slight or deep constrictions[43].
I must next say something of the ganglions[44]. Lyonet has observed that, in the caterpillar of the great goat-moth, these in one respect differ remarkably from the chords that connect them; in the latter the air-vessels or bronchiæ only cover the outside of the tunic, while in the former they enter the substance of the ganglion, which is quite filled with their delicate and numberless branches[45]. Every ganglion may be regarded in some degree as a centre of vitality or little brain[46], and in many cases, as well as the brain, they are formed of two lobes[47]. I shall now consider them more particularly as to their station, number, and shape.
1. With regard to the first head, their station, they are most commonly divided between the trunk and abdomen; but in some cases, as in Hydrophilus piceus and Acrida viridissima, the first ganglion is in the head[48]; in others, as in the louse, the water-scorpion, and the grub of the rhinoceros-beetle, they are confined to the trunk, their functions in the abdomen being supplied by numerous radiating nerves[49]; in others again, as in the scorpion, they are all abdominal. The ganglions vary also in their situation with respect to each other. Thus in some, as in the larva of the Chamæleon-fly (Stratyomis Chamæleon), they are so near as to appear like a string of beads[50]; in that of the ant-lion (Myrmeleon) the two ganglions of the trunk are separated by an interval from those of the abdomen, which are so contiguous as to resemble the rattle of the rattle-snake[51]. In others the internodes are longer, and the ganglions occur at nearly equal intervals, as in the larva of the Ephemeræ[52]; but in the majority they are unequal in length: thus in the scorpion the three first ganglions are the most distant[53]; in the hive-bee the third and fourth[54]; and in the spider the last[55].
2. The ganglions also in different species, and often in the same insect in its different states, vary in their number. Thus in the grub of the rhinoceros-beetle the whole spinal marrow appears like a single ganglion divided only by transverse furrows[56]; in the water-scorpion there are two[57]; in the louse there are three[58]; in the rhinoceros-beetle there are four[59]; five in the stag-beetle[60]; seven in the hive-bee and some Lepidoptera[61]; eight in the grub of the stag-beetle[62]; nine in the great Hydrophilus[63]; ten in Dytiscus[64]; eleven in the grub of the great Hydrophilus[65]; twelve in the grub of Dytiscus and the caterpillars of Lepidoptera[66]; thirteen in the larva of Æshna[67]; and twenty-four in Scolopendra morsitans[68]. You must observe that, generally speaking, the number of ganglions is less in the imago than in the larva. With regard to the distribution of these knots to the different primary parts of the body, the following table will exhibit it, as far as I am acquainted with it, at one view. I omit those in which the ganglions are only in one of these parts.